1. Field of the Invention
The present invention relates to a novel molding method and device for use in measuring the bonding strength of elastomeric compositions to various prepared surfaces.
The advent of complex and high density electronic systems for ground, airborne and space applications has required the parallel development of packaging technology including the use of elastomers to protect electronic system from vibration, shock, temperature extremes, and as a dielectric medium to prevent voltage breakdown between the internal components or to the containment vessel walls. Meeting these requirements depends upon maintaining a viable bond between the elastomer and substrates, components and containment walls. This bond strength depends on the elastomer and the physical and/or chemical preparation of the bonding surface. Therefore accurate quantification of this bond strength is critical to the development of these new systems to withstand the dynamic forces and thermal contraction which place significant tension stresses on the interface between the elastomer and the surface to which it is bonded.
2. State of the Art
Previously, lap shear tests have been relied upon to assess the bond strength of elastomers with various surfaces. However, the physics of the interface dictate that the relationship between the lap shear test results and the ability to withstand tensile forces at the surface is in itself a function of the surface/elastomer bond, resulting in the need to measure the tensile bond strength directly. Classical methods which measure the tensile bond strength successfully are accurate only when the bonding material is fairly rigid and low strains are induced during the measurement. In this case, most of the loading is uniaxial, with the average and local internal stress in the material very close, and the bond fails at substantially below the tensile limit of most rigid test materials. However, in the case of elastomers, if there is a significant thickness of material between the elastomer support structure and the test sample, at typical bond strengths obtainable, significant triaxial strains and stresses occur in the elastomer due to the stretching or necking down effects which cause rupture failures in the elastomer prior to reaching critical stresses at the interface. Thus valid results are not obtainable. With this background and in light of the limitations of the prior art, the present invention was conceived.
A variety of different devices are known for determining the bonding strength of various adhesives, coatings and paints for various surfaces by compressing a coated substrate and then measuring the bonding strength as the ultimate separation force that the bond can resist after the adhesive is set or cured. Reference is made to the following U.S. Pat. Nos. for their disclosure of such devices: 3,628,378; 3,821,892; 4,567,758; 4,586,371; 4,606,225; 4,893,513, 5,313,841; 5,361,639; and 5,649,447.
None of the aforementioned patents deal with or solve the problem of the test material developing significant triaxial stress risers, or necking down, under the effects of the tension forces generated during separation, and therefore the devices of these patents are unsatisfactory for the accurate measurement of the bonding strength of non-rigid, rubbery elastomers to prepared surfaces.